Iron oxyhydroxide colloid formation by gamma-radiolysis.

Gamma-irradiation of deaerated aqueous solutions containing FeSO4 leads to the formation of uniform-sized colloidal particles of gamma-FeOOH. At short irradiation times, or in solutions with a low initial [Fe2+](0), spherical particles with a size less than 10 nm are formed. These primary particles grow to form a dendritic structure upon longer irradiation, and the final size of the large particles is similar to 60 nm with a very narrow size distribution. Further prolonged irradiation does not change the final particle size. The narrow size distribution is attributed to rapid homogeneous radiolytic oxidation of soluble Fe2+ to relatively insoluble Fe3+ hydroxides [Fe(H2O)(6-n)(OH)(n)](3-n) leading to particle nucleation by spontaneous condensation. These primary particles then grow into gamma-FeOOH particles with a dendritic structure. The final size reached at long times is regulated by the steady-state redox conditions established during long-term irradiation at the aqueous-solid interface.